Which transponder code is used to indicate an in-flight emergency?

The transponder code used to indicate an in-flight emergency is 7700. This specific code is universally recognized in the aviation community as a distress signal. When a pilot selects this code on their transponder, it alerts air traffic control and other aircraft in the vicinity that an emergency situation has arisen, prompting immediate assistance and priority handling.

The codes serve different purposes: 7500 is for indicating unlawful interference, such as hijacking, while 7600 signals a loss of communications. The code 7800 is not assigned for any specific emergency purpose in standard aviation operations. Understanding these distinctions is crucial for proper communication and coordination in emergency situations.